The following abbreviations are herewith defined, at least some of which are referred to within the following description.
3GPP Third Generation Partnership Project
ACK Positive-Acknowledgment
BLER Block Error Ratio
BPSK Binary Phase Shift Keying
CAZAC Constant Amplitude Zero Auto Correction
CCA Clear Channel Assessment
CCE Control Channel Element
CP Cyclic Prefix
CQI Channel Quality Information
CSI Channel State Information
CSS Common Search Space
CWS Contention Window Size
DCI Downlink Control Information
DL Downlink
eCCA Enhanced Clear Channel Assessment
eNB Evolved Node B
EPDCCH Enhanced Physical Downlink Control Channel
ETSI European Telecommunications Standards Institute
FBE Frame Based Equipment
FDD Frequency Division Duplex
FDMA Frequency Division Multiple Access
FEC Forward Error Correction
HARQ Hybrid Automatic Repeat Request
LAA Licensed Assisted Access
LBE Load Based Equipment
LBT Listen-Before-Talk
LTE Long Term Evolution
MCL Minimum Coupling Loss
MCS Modulation and Coding Scheme
MU-MIMO Multi-User, Multiple-Input, Multiple-Output
NACK or NAK Negative-Acknowledgment
OFDM Orthogonal Frequency Division Multiplexing
PCell Primary Cell
PBCH Physical Broadcast Channel
PDCCH Physical Downlink Control Channel
PDSCH Physical Downlink Shared Channel
PHICH Physical Hybrid ARQ Indicator Channel
PRACH Physical Random Access Channel
PRB Physical Resource Block
PUCCH Physical Uplink Control Channel
PUSCH Physical Uplink Shared Channel
QoS Quality of Service
QPSK Quadrature Phase Shift Keying
RAR Random Access Response
RRC Radio Resource Control
RX Receive
SC-FDMA Single Carrier Frequency Division Multiple Access
SCell Secondary Cell
SCH Shared Channel
SIB System Information Block
SINR Signal-to-Interference-Plus-Noise Ratio
SR Scheduling Request
TBS Transport Block Size
TDD Time-Division Duplex
TDM Time Division Multiplex
TX Transmit
UCI Uplink Control Information
UE User Entity/Equipment (Mobile Terminal)
UL Uplink
UMTS Universal Mobile Telecommunications System
WiMAX Worldwide Interoperability for Microwave Access
In wireless communications networks, LAA facilitates an LTE system to use an unlicensed spectrum with assistance from licensed carrier. LAA further aims to facilitate the fair coexistence with other technologies over the unlicensed spectrum and to satisfy various regulatory requirements in different countries and regions. As stated in LAA SI, TR36.889. an LBT procedure may facilitate fair and friendly coexistence of LAA with other operators and technologies operating in the unlicensed spectrum. In TR36.889, various LBT schemes are defined, including a category 4 relating to LBT with random back-off with a contention window of variable size. Specifically, TR36.889 states for category 4 that “[t]he LBT procedure has the following as one of its components. The transmitting entity draws a random number N within a contention window. The size of contention window is specified by the minimum and maximum value of N. The transmitting entity can vary the size of the contention window before drawing the random number N. The random number N is used in the LBT procedure to determine the duration of time that the channel is sensed to be idle before the transmitting entity transmits on the channel.”
In certain configurations, a contention window size (“CWS”)may be updated after the completion of each downlink transmission burst. In other configurations, the CWS for LBT category 4 channel access scheme in an unlicensed carrier may only be increased in situations with high load in order to avoid the channel collision or decreased in situations with low load in order to improve spectrum efficiency. For each adaptive adjustment, LAA base units (e.g., eNBs) may facilitate various equipment having a fair share of unlicensed spectrum access opportunities (e.g., for itself and for all the scheduled remote units (e.g., UEs)). In the meantime, base units may also aim to adjust the CWS to increase the overall unlicensed spectrum utilization efficiency and reduce the probability of transmission collision.
There are various different interference situations involving base units and remote units. In some situations, a base unit may encounter a problem with hidden nodes. One example is shown in a wireless communication system 700 illustrated in FIG. 7. The wireless communication system 700 includes a base unit 702 (e.g., an LAA eNB) having a CCA range 704 and a cell coverage area 706. The base unit 702 transmits downlink data to its served remote unit 708 (e.g., UE1). Another node, which in this example is a Wi-Fi AP 710, is located nearby to the remote unit 708, but the Wi-Fi AP 710 transmissions cannot be sensed by the base unit 702. In this case, for DL PDSCH transmission to the remote unit 708, the CWS may benefit from being increased to avoid possible collision with Wi-Fi transmission. However, given that the base unit 702 is unable to detect the presence of the Wi-Fi AP 710 based on the CCA energy detection or preamble detection due to coupling loss between the base unit 702 and the Wi-Fi AP 710, the base unit 702 may actually decrease its CWS for fast channel access assuming. Consequently, transmission collision between the base unit 702 and the Wi-Fi AP 710 may occur.
In another configuration, the CWS may be adjusted based on HARQ-ACK feedback. A similar mechanism is used in Wi-Fi: if ACK is not received after a frame from a station, a Wi-Fi AP determines that a collision happened and therefore doubles the CWS; otherwise, Wi-Fi AP resets the CWS to the minimum value. It is noted that in Wi-Fi, a transmission burst is only for a single remote unit or station. On the other hand, for LAA, it is possible that a transmission burst may include data transmitted to more than one remote unit. Therefore, this ACK/NACK based triggering mechanism has some drawbacks for LAA, especially in the case that a base unit schedules multiple remote units in one DL transmission burst or even schedules multiple remote units in one single subframe. ACK/NACK corresponding to each remote unit's DL data in each DL subframe will be reported to the base unit. Therefore, adapting the CWS based on the HARQ-ACK from all scheduled remote units collectively may not reflect the remote unit-specific interference situations present to properly adjust the CWS. One example is shown in a wireless communication system 800 illustrated in FIG. 8. The wireless communication system 800 includes a base unit 802 (e.g., an LAA eNB) having a CCA range 804 and a cell coverage area 806. The base unit 802 transmits downlink data to its served remote units 808 (e.g., UE1) and 810 (e.g., UE2). Another node, which in this example is a Wi-Fi AP 812, is located nearby to the remote unit 808, but the Wi-Fi AP 812 transmissions cannot be sensed by the base unit 802 or the remote unit 810. The remote unit 808 suffers the interference from the hidden node of Wi-Fi AP 812, while remote unit 810 has no hidden node problem. Therefore, serving remote units 808 and 810 may use different CWS for optimal performance.